dsilvercoin,
First, sorry pal but this is a long reply from a long winded guy. Second, I've replied assuming you're going to use this for film or t.v. scoring scenarios, whether mockup or production use.
Long term investment and believability (realism in sound) are always the most important traits to any library for me. VSL and Spitfire are the ONLY two libraries that have ever lived up to my standards for realism. With that in mind, one cannot ignore that Spitfire has a highly restrictive license and libraries. The expensive stuff requires that you regularly hire performers (which my being in the 'I'm pursuing a career' stage in my life, does not function within their policy). The affordible stuff has a good sound, but 1) It's nothing I can't obtain from VSL. It's just a good 'out of the box' sound, meaning less tweaking required. Also, 2) that 'less tweaking' point is paramount to me. The lower cost libraries offer no flexibility and very limited articulations. One can only make something sound believable with such limitations.
Another point is software. Spitfire has no desire to persue software development. While I used to love Kontakt (and still find occasional uses for it), Vienna Instruments Pro 1.0 blew Kontakt out of the water. It truly was a night and day difference for my productivity and ease of use. Yes, there was a learning curve. To get my samples sounding how I want, I had to put forth a bit of hard work and effort. But now that I do, my only owning the Special Edition, SE Strings Plus, Fanfare Trumpets, and Epic Horns dwarf any other library's ability to sound real. VI Pro 2.0 is now out and it is even more phenominal. The enveloped time stretching has given me tremolo and trill possibilities previously unavailable. I now have performances that no other library can even acheive.
By all means, Spitfire is good. If you want quick and easy and one style only, I'd say go for it. But if you want to actually write anything according to your composing choices, style, and so on- AND if you want to write for multiple styles of music... I'd say VSL is the far more logical choice. In the end all of those points boil down to investment to me. My priority is VSL. Why? Because I can do anything with it and sound better than any other library available. Because I am not limited by wet samples. Because their software allows for far more productive approaches than Kontakt could ever dream.
In the end, it's better to own both, everything even. It's even better to record real performances. But for the time and money, it's VSL. For the flexibility and quality of sound, it's VSL. I may sound a bit too VSL fanboy-ish but it is only because of my good experience. I could care less about asthetics and simpleton way that some companies approach things. I care about function above all else. VSL functions best imo. So the cost is far more justified.
Hopefully that helps,
-Sean
P.S. I had a friend explain how much he loved Hollywood Strings. I explained that 'There is no such thing as a Hollywood Cello'. What one needs in the industry is not the same sound that everyone else is producing. No one wants that. What they want is the ability to do that sound, other sounds, richer, darker, louder, and every other kind of sound one can imagine. When in film scores I hear dry, wet, and every other combination of everything out there... I can't help but remember that VSL is dry, and no one else really is. I can do both, they can't. That may seem like a simple answer, but in the end... what is more effective and what is going to give you the flexibility you need to produce anything asked of you?